Conventionally, ink-jet printers include a printhead positioned above a print medium during printing. The print medium can also be referred to as print material. For purposes of this disclosure, the print material can be mylar, paper, cardboard, envelope material, or any other sheet material. A support structure, usually a platen, is positioned below and typically supports the print medium during printing. During printing, ink from the printhead is printed on the print material to form the desired image. However, ink from the printhead wets the fibers of the print material, causing the print material to buckle or curve. As the ink density, or amount of ink, printed on a page increases, the amount of bending or curving increases. Because the platen is positioned directly below the print material, the print material tends to buckle or curve upwardly instead of downwardly away from the printhead. This upward buckling increases the possibility of the print material contacting the printhead, smearing the freshly printed ink on the print material.
This buckling, or deformation, creates many additional problems. For example, the deformation creates an unappealing appearance of the final documents. In addition, as the paper deforms, the distance between the paper and the ink-jet pens, also called head-to-print material spacing, across the width of the page varies. Due to this uneven spacing during printing, the ink droplets are not evenly applied to the print material from the same distance. To achieve high quality print images, the head-to-print material spacing in an ink-jet printer should be maintained at approximately 1-to-1.5 millimeters or lower. This is relatively easy at lower ink densities, but difficult to maintain at higher densities. Thus, the uneven spacing due to print medium bending causes severe problems in the final print quality of the sheet.
In the past, in an effort to reduce these types of paper bending, printers have included high-powered heaters to drive off moisture. However, incorporating a high-powered heater into a printer adds to the complexity and to the cost of the printer mechanism. The heater also creates a fire and burn safety problem. Additionally, incorporation of a heater in a printer decreases throughput because extra time is required to drive moisture from the print material. Incorporation of heaters also causes print image distortion problems because the print medium unevenly shrinks during drying.
Other printers have included manually adjustable printheads so that the printhead is moved upwardly to compensate for an upward bend of the print material. Manual adjustment requires operator labor and therefore decreases efficiency. The additional operator labor also reduces print quality due to the possibility of operator error such as adjusting the pen in too low a position which would allow the curved paper to impact the pen.
Therefore, there is a need for a wet ink printer mechanism which reduces upward buckling of the print medium by providing a platen which moves downwardly away from the print medium during printing of relatively high ink density to ensure a relatively controlled head-to-print material spacing.
There are several variable support mechanisms which are moveable away from the print material. Kwan, U.S. Pat. No. 3,995,730, describes a moveable platen which is retractable so that an operator can insert a noncontinuous form, such as punch cards or multiple section forms. Rasmussen, U.S. Pat. No. 4,728,963, describes a moveable platen in an ink-jet printer setting. The Rasmussen platen supports the paper throughout the printing process. At the end of the process, the platen pivots downwardly, thereby eliminating undesirable clamping of the sheet of print material between the platen and a paper guide. Once the Rasmussen platen is moved away from the paper guide, the sheet is free to drop into a paper output tray. These two patents both describe a moveable print material support. However, neither describes a moveable platen which retracts during printing in response to measured ink density on the print material.